Equipotential cathode for electric discharge tubes



1960 H. G. GERLACH ETAL 2,949,557

EIQUIPOTENTIAL CATHODE FOR ELECTRIC DISCHARGE TUBES Filed 001;. 3, 1958 2 Sheets-Sheet 1 NVENTOR HAN S GEORG GERLACH EDUARD GERHARDUS DORGE LO Aug. 16, 1960 H. G. 'GERLACH ET AL 2,949,557

EQUIPOTENTIAL CATHODE FOR ELECTRIC DISCHARGE TUBES Filed 001:. s; 1958 2 Sheets-Sheet 2 INVENTOR HANS GEORG GERLACH EDUARD GERHARDUS DORGELO AGENT EQUIPOTENTIAL 'CATHODE FOR ELECTRIC DISCHARGE TUBES Hans Georg Gerlach and Eduard Gerhardus Dorgelo, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Oct. 3, 1958, Ser. No. 765,254 Claims priority, application Netherlands, Oct. 4, 1957 6 Claims. (Cl. 313-346) This invention relates to tungsten-thorium equipotential cathodes and more particularly to such cathodes having a smooth surface. It also relates to electric discharge tubes, more particularly for short waves, containing such cathodes.

It is known a tungsten-thorium equipotential cathode may be manufactured by helically winding a wire of this material so that the returns engage one another. The cathode was then covered with tungsten powder and sintered, resulting in a solid tubular body being obtained. By carrying out the sintering process in a special way, it is possible to obtain a structure having a surface as smooth as possible.

However, such a cathode has the disadvantage that the sintered cylindrical surface is not sufliciently smooth for very high frequencies. Furthermore, the cylinder is electron-emissive not only on the outer side, but also on the inner side where the heater is situated.

These disadvantages are fully avoided if, according to the invention, the engaging turns of the wire of the tungsten-thorium cathode are wholly covered with a solder on the side adjacent the heater and ground off to half their thickness on the electron-emissive side, so that the cathode has a continuous smooth surface.

The solder may consist of the carbide of tungsten, molybdenum, tantalum, orthorium which has a smaller electron-emitting capacity than tungsten-thorium, a melting point of more than 2000 C. and a low vapour pressure. It is possible to add to the carbide a refractory metal in the pulverulent form, for example powdery tungsten by simply mixing the powders with nitro-cellulose, binder to form a paste in order to enhance the roughness of the surface, so as to obtain a good transmission of heat from the heater to the surface of the cathode. The amount of W powder is not critical and may be about equal to the amount of carbide.

A suitable paste consists e.g. of 100 g. tungsten-carbide (WC) powder, to which is added a binder, consisting of 30 cc. acetone, 30 cc. diacetone-alcohol and 30 cc. nitrocellulose.

In order that the invention may be readily carried into efiect, it will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a tube containing a cathode according to the invention of a determined embodiment;

Fig. 2 shows a detail of the cathode cylinder, and

Figs. 3 and 4 show other embodiments of the cathode.

In Fig. 1, reference numeral 1 indicates the bulb of the tube which is closed by a base 2, into which contact pins 3 are sealed. One of the pins 3 carries rod 4, the upper end 9 of which is connected to a heater 5 and an equipotential cathode 6. The cathode 6 is surrounded by a grid 7 and an anode 8.

The cathode 6, as shown in detail in Fig. 2, consists of a tungsten-thorium wire 10 of the usual type employed to make thoriated tungsten cathodes for transmitting type States Patent wound to form a cylinder having a diameter of, for example, 1 cm. and a length of 3 cm., which is covered with soldering material 11.

After the soldering material 11, which preferably consists of a carbide of a refractory metal, preferably Mo, has been melted, the outer side of the cylinder is ground ofi to half the thickness of the tungsten thorium wire, resulting in a continuous, perfectly smooth tungsten-thorium surface. On the inner side, the surface is constituted by the soldering material 11, which is rough and has a low electron-emitting capacity. Due to the smooth outer surface, the radiation of heat is small, whilst the inner side of the cathode cylinder well absorbs the radiation of heat from the filament 5. The cathode, after being ground off, may be carbonized in the usual manner by heating in a vapour of a carbon-hydrogen compound. Dueto the comparatively large wall thickness, the cathode is very well resistant to mechanical shocks. If desired, the inner surface of the cylinder may be the electron emitting surface.

In Fig. 3, the cathode, which in this case is flat, consists of a tungsten-thorium wire 3, likewise of, for example, 1 mm. in diameter, which is spirally wound to form a disc having a diameter of, for example 1 cm, the disc on the side adjacent the filament 14 being covered with the soldering material 11 and soldered to :a supporting cylinder 12. The filament 14 is mounted on an insulating body 15, for example of ceramic material, which is secured in the supporting cylinder 12. The cylinder 12 may be secured in a discharge tube by means of a flange 13. The heating may take place wholly or in part by bombardment by electrons, if a positive potential with respect to the heating wire 14 is applied to the cathode 10. This is convenient, more particularly for cathodes of large diameter (5 cms.). The heating wire, which consists of tungsten, then constitutes together with the lower side of the cathode, a diode.

In Fig. 4, the electron-emissive surface of the cathode is cup-shaped, so that a concentration of electrons may be obtained.

Although a few determined embodiments have been described, the cathode according to the invention may alternatively have other shapes, for example a conical or spherical shape.

What is claimed is:

1. An indirectly heated equipotential cathode for an electric discharge tube comprising a body formed by a plurality of turns of tungsten thorium wire, adjacent turns of said wire being spaced by a distance substantially less than the diameter of said wire and having adjacent coplanar portions for providing a substantially continuous smooth tubular surface on the outer side of said body, and a layer of a refractory metal solder covering the inner surface of the body and extending between adjacent turns of the wire.

2. A cathode as claimed in claim 1, in which the solder consists essentially of a carbide of a refractory metal.

3. A cathode as claimed in claim 2 in which the solder also contains a refractory metal in the powdery state.

4. A cathode as claimed in claim 3, in which the oathode has the form of a cylinder consisting of a helicallywound wire, the inner side of which is coated with the soldering material and the outer side of which is a substantially smooth continuous tungsten-thorium surface.

5. A cathode as claimed in claim 3, in which the surface of the cathode has the form of a disc.

6. A cathode as claimed in claim 3, in which the surface of the cathode is cup-shaped.

References Cited in the file of this patent UNITED STATES PATENTS Warren Oct. 17, 1950 D6 wanww-uw-nmnun 8Q 

